We report on the facile synthesis of aligned single-walled carbon nanotube (SWNT) arrays with controllable density on A-face sapphire substrates and the measurement of their polarized Raman spectra. The catalyst was prepared by simply dipping the sapphire substrate into a mixed solution of iron nitrate and molybdenum acetate, where Mo improved the nanotube yield as well as reproducibility. The angle dependence of the Raman spectra measured for the high-density nanotube array indicated the growth of SWNTs aligned mainly in the [110 0] direction and two other directions. This suggests multiple mechanisms for the alignment, such as the pseudo one-dimensional Al arrangement and the presence of shallow grooves. (c) 2006 Elsevier B.V. All rights reserved.